Process for making sodium sulphate and chlorine



May 1, 1945.

R. K. ILER ET'AL PROCESS FOR MAKING SODIUM SULPHATE AND CHLORINEOriginal Filed Dec. 25, 19:59

Oleum Pea d0? Oleum J03 Still 212503 Weak Acid Na; .50

-- 75 Recovery Weak 75 Chlorine Acz'la Refovezy Cazalyzz'c Gleam 672 $03Converter 02 AbSOI'I 25o .0 250 Ralph Kller INVENTOR Francis Jf4damPatented May-1, 19 45 Ralph K. Iler, East-Cleveland, and Francis J. Mc-

Adam, Cleveland, Ohio; assignors to E. I. du

Pont de Nemours & Company,

Wilmington,

Del., a corporation of Delaware Original application December 23, 1939,Serial No.

310,838. Divided and this application November 29, 1940, Serial No.367,745

4 Claims. (Cl- 23-219) This invention relates to the manufacture ofchlorine and is directed particularly to the recovery of chlorine frommixtures of sulphur dioxide and chlorine as obtained by the action ofsulphur trioxide on sodium chloride. More particularly this invention isdirected to processes in which sulphur trioxide obtained from a sulphurtrioxide-addition complex is caused to act on sodium chloride amounts ofsulphur dioxide and chlorine, the

. sulphur dioxide oxidized to sulphur trioxide, the sulphur trioxideseparated by combination with a, substance adapted to form therewith asulphur trioxide-addltion complex, and decomposing said addition complexeither to liberate sulphur trioxide for reaction with sodium chlorideor, in the case of the NaCl complex, to liberate sodium sulphate,chlorine, and sulphur dioxide.-

It has long been recognized as desirable to produce chlorine and sodiumsulphate by reacting sulphur trioxide with common salt, and diversprocesses hav been suggested foraccomplishing this end. Yet, the-factthat none of these suggestions have proved of practical value suggeststhat in some aspects at least the processes were impractical, and w findthat one of the reasons lies in the failure of the prior art to provideany practical method of separating the equimolecular mixtures of sulphurdioxide and chlorine which result from the reaction of salt and sulphurtrioxide, either directly at. appropriate temperatures or through thedecomposition of an intermediateproduct, sodium chlorosulphonate. Thus,

to liberate 'equimolecular it has been proposed to makechlorine and salta cake (sodium sulphate) by passing sulphur trioxide through towerspacked with salt, passing the resulting mixture in the presence of airor oxygen in contact witha-material adapted to catalyze the reactionsulphur dioxide to sulphur trioxide, passing the resulting mixture againin contact with sodium chloride whereby the sulphur trioxide is causedtoreact with the salt to form more sodium sulphate, sulphur dioxide,chlorine, and so on; until the sulphur dioxide in the gas is reduced toa nominal figure. Such processes, however, cannot produce a sulphurdioxide-free gas because each step for the removal of sulphur trioxideis accompanied by the formation of sulphur dioxide. It has also beenproposed to separate sulphur dioxide and chlorine by liquefaction andfractional distillation, but complete separation is diificult to obtainand the impure sulphur dioxide is of little value by itself.

We have now found that chlorine and sodium sulphate may be producedsimply and effectively by causing sulphur trloxide to act on sodiumchloride in a manner to produce a gaseous mixture composed essentiallyof equimolecular quantitles of sulphur dioxide and chlorine, addingoxygen to the mixture, catalytically oxidizing the sulphur dioxide tosulphur trioxide, and separating the sulphur trioxide as a sulphurtrioxideaddition complex which may be utilized in the production offurther quantities of sulphur d1- oxide and chlorine.

Our invention may be more fully understood by referring to theaccompanying drawing which illustrates graphically the several stepsutilized in carrying out the processes of our invention. As thusillustrated, we first cause sulphur trioxide to act on salt in a mannersuch that the products are sodium sulphate, sulphur dioxide, andchlorine. This reaction may be carried out in asingle step at atemperature above about 450 C., in which case the reaction may be represented as 2NaCl+2SO3- Na2SO-i+SO2+CIz or the reaction may be carried outin two stages in which sodium chlorosulphonate is first formed byreacting sulphur trioxide and salt at a temperature not substantiallyexceeding 150 ,C. and preferably below about C., and then decomposed byheating into sodium sulphate, sulphur dioxide and chlorine.

Sulphur trioxide suitable for these reactions may be obtained in theusual manner by the oxidation of sulphur in thecontact process. Theconverter gases from such processes are freed of undesirableconstituents by absorbing the sulphur trioxide in sulphur trioxidemonohydrate to form oleum or fuming sulphuric acid from which thesulphur trioxide may be regenerated in a highly pure form. It isespecially desirable to have ahighly purified sulphur trioxide if thehigh temperature process for the direct foimation of sodium sulphate;sulphur dioxide, and chlorine is employed, because otherwise the gaseousproducts of the reaction will be contaminated with the undesirableconstituents ofthe sulphur trioxide. ployed, however, less highlypurified sulphur trioxide may be used since in this case the sulphurtrioxide is wholly absorbed as sodium chlorosulphonate and theimpurities are thereby separated.

The sulphur dioxide-chlorine mixture thus obtained is diluted withoxygen in the amount required to givethe desired S0220: ratio foreflicient conversion of the sulphur dioxide to sulphur trioxide and somuch diluent gas, prefer- If the low temperatur reaction is em- This gasmixture isthen exposed to a catalyst or oxidation promoter underconditions adapted to promote the oxidation of sulphur dioxide tosulphur trioxide.

The amount of oxygen introduced should be kept to a minimum since anyunreacted oxygen passing through the converter will have to be separatedin a subsequent step. Preferably, oxygen is added in an amountsufficient that the molai ratio of sulphur dioxide to oxygen is notsubstantially less than one and not substantially greater than two. Itmay be desirable, however. in some cases to operate with a deficiency ofoxygen, especially since small amounts of sulphur dioxide may be morereadily separated from chlorine than small amounts of oxygen.

The gas mixture emanating from the converter will consist predominantlyof sulphur trioxide and chlorine together with minor amounts ofoxygenand/or sulphur dioxide. The composition of'this mixture willdepend not only upon the sulphur dioxide to oxygen ratio and theefilciency of the converter but also upon the amount of diluent gas(recycled chlorine) introduced. In general, the components of theconverter gas other than sulphur trioxide and chlorine will not exceedmore than about 25 per cent, by volume, and preferably these componentsare desirably held to less than about 5 per cent by volume.

The gases emanating from the converter after being suitably cooledarecontacted with a substance adapted to form an addition complex withsulphur trioxide. By sulphur trioxide-addition complexes we meansubstances which are formed by simple addition reactions between sulphurtrioxide and some other compound. Compounds which so react with sulphurtrioxide are well known to those skilled in the art, and as exam-Instead of using sodium chloride as the absorbent to separate sulphurtrioxide from the converter gases we have found it of advantage toemploy adsorbents in which the sulphur trioxide is loosely bound and canbe regenerated. We may, for example, pass the converter gases afterbeing suitably cooled, say to about to 100 C., in contact with sulphuricacid to form oleum or fuming sulphuric acid. The absorbent in such caseis the sulphur trioxide monohydrate and may be regenerated on theapplication-of heat sufllcient to drive off the absorbed sulphurtrioxide and recycled for separating further quantities of $111. phurtrioxide from the converter gases. The regenerated sulphur trioxide maybe recycled to the sodium chloride sulphur trioxide reaction, but forthe most part it will be found more desirable to combine the oleum thusproduced with oleum from an extraneous source, as for example a contactsulphuric acid plant, and to strip the sulphur trioxide therefrom in asingle oleum still. In place of sulphuric acid other absorbents such assodium sulphate, which forms sodium pyrosulphate, may be used,

In view of the highly reactive nature of mixtures of sulphur dioxide andchlorine, and particularly in view of the tendency of such mixtures toreact with water to form sulphuric acid and hydrochloric acid, itisdesirable that a high degree of efiiciency in the conversion of thesulphur dioxide to sulphur trioxide be obtained if the absorbentcontains any substantial quantity of water. We have found that thisdifliculty may be avoided by using a vanadium catalyst, since with thiscatalyst a sufllciently high percentage conversion can be obtained thatvery little chlorine will be lost to the process even if water is pieswe may cite sulphur trioxide monohydrate,

sodium sulphate, and sodium chloride.

The gases leaving the converter, for example, may be cooled tobelow 150C. and preferably to about to 100 C. and passed-in contact with sodiumchloride, preferably in the presence of a small quantity of hydrogenchloride as a catalyst, and the sulphur trioxide will be absorbed as thesulphur trioxide-addition complex sodium chlorosulphonate. The chlorinepasses on unchanged and may be freed or small amounts of sulphur dioxideor sulphur oxychloride, oxygen, andanv other impurities present in anysuitable manner.

The sulphur compounds present may be removed, for example, by scrubbingwith water or dilute acids, or by fractional distillation. Owgen too maybe separated by fractional distillation, by.

preferential adsorption, or in any other suitable manner.

The sodium chlorosulphonate so formed may be recycled to the salt andsulphur trioxide reaction andthere decomposed to form additionalquantities of sodium sulphate, chlorine, and sulphur dioxide. The use ofsalt as an absorbent may advantageously be employed in such processes asemploy two steps in the salt sulphur trioxide reaction, since in suchcase sodium chlorosulphonate is a product of the first step of thereaction and also a product of the final step of the process. The twoseparate lots or sodium chlorosulphonate can be combined in a singlestep of decomposition to sodium sulphate, chlorine, and sulphur dioxide.

present in the absorbent. We. have also found that loss of chlorinemaybe avoided in this manner by effecting the absorption of sulphurtrioxide from the converter .gases in sulphur trioxide monohydrate. Itappears that in an absorbent in which the sulphur trioxide is in anamount at least molecularly equivalent to the water the tendency ofsulphur dioxide and chlorine to react with the water to formhydrochloric acid and sulphuric acid is avoided.

This application is a-division of application Serial No. 310,838, filedDecember 23, 1939, of which Ralph K. Iler is now the sole applicant.

We claim:

1. In'the manufacture of chlorine and sodium sulphate the method whichcomprises heating oleum to driveofl sulphur trioxide, causing the,sulphur trioxide which is thus obtained in the essentially pure stateto react with sodium chloride in a manner such that the products aresodium sulphate, sulphur dioxide and chlorine, isolating the solidproducts of the reaction from the gaseous mixture ofsulphur dioxide, andchlorine, treating said gaseous mixture catalytically to convert itssulphur dioxide to sulphur trioxide, treating the resulting mixture toseparate the chlorine and-the sulphur trioxide, recovering the sulphurtrioxide as oleum and recycling the oleum to the first-named step.

2. In the manufacture of chlorine and sodium sulphate the method whichcomprises subjecting sulphur dioxide-containing gas. to catalysis underconditions adapted to form sulphur trioxide, passing the sulphurtrioxide-containing gas in contact with sulphur trioxide monohydrateunder conditions adapted to promote the'formation of fuming sulphuricacid thereby to form oleum,

which is thus obtained in the essentially pure state to react withsodium chloride in a manner such that the products are sodium sulphate,sulphur dioxide and chlorine, isolating the solid products of thereaction from the gaseous mixture of sulphur dioxide and chlorine,treating said gaseous mixture catalytically to convert its sulphurdioxide to sulphur trioxide, treatingfithe resulting mixture to separatethe chlorine and ous mixture of sulphur dioxide and chlorine,

treating said gaseous mixture catalytically to convert its sulphurdioxide to sulphur trioxide, treating the resulting mixture to separatethe chlorine and the sulphur trioxide, said S paration being efl'ectedby absorption and regeneration of the sulphur tllOXidGp and, recyclingthe'sulpliur trioxide thus recovered for. reaction with sodium chloride.

4. In the manufacture of chlorine and sodium sulphate the 'method whichcomprises causing sulphur trioxide to react with sodium chloride in amanner such that the products are sodium sulphate, sulphur dioxide andchlorine, isolating the solid products or the reaction from the gaseousmixture of sulphurdioxide and chlorine.

treating said gaseous mixture catalytically to convert its sulphurdioxide to sulphur trioxide, treating the resulting mixture to separatethe chlorine and the sulphur trioxide, said separation" being effectedby absorption and regeneration of the sulphur trioxide in sulphurtrioxide monohydrate, and recycling the sulphur trioxidethus recoveredfor reaction with sodium chloride. Y I

RALPH K. ILER. FRANCIS J: McADAM.

